1. Field of the Invention
The invention relates to an X-ray examination apparatus in which the influence of the X-ray filter on the adjustment of the X-ray examination apparatus is substantially mitigated.
2. Description of the Related Art
Such an X-ray examination apparatus is known from European patent application EP 0 629 105.
The known X-ray examination apparatus comprises an X-ray image intensifier and an image pick-up device. The X-ray image intensifier converts the X-ray image into an optical image. The image pick-up device derives the image signal from the optical image. The brightness control system of the known X-ray examination apparatus comprises an auxiliary photodetector which measures brightness values of a portion of the optical image. This portion is called the measuring field. The brightness control system is arranged to adjust the X-ray examination apparatus on the basis of the measured brightness values of the measuring field. In particular, the energy of the X-rays from the X-ray source is adjusted by the brightness control system. Although the measuring field of the known X-ray examination apparatus is adjustable, it has been found that it is difficult to avoid that a part of the X-ray filter is imaged in the measuring field. Brightness values in the measuring field which correspond to the X-ray filter lead to a sub-optimum adjustment of the X-ray examination apparatus.
The X-ray filter is, for example, adjusted so as to attenuate X-rays which are hardly attenuated by the object to be examined. Notably when a radiological examination of the patient's spinal column is performed, a part of the X-rays which pass through lung tissue are hardly attenuated by the lung tissue which contains a comparatively large amount of air. In such a radiological examination, the X-ray filter is positioned such that the X-rays that are directed towards the portion of the lungs of the patient are attenuated comparatively strongly by the X-ray filter and X-rays that are directed to the patient's spinal column are hardly or not at all attenuated by the X-ray filter. Accurate positioning of the X-ray filter is disclosed in European patent application EP 0 496 438. It has been found, however, that detrimental effects of the adjustment of the X-ray examination apparatus can occur despite of accurate positioning of the X-ray filter.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.